The hypereutectic aluminum-silicon alloys have been produced by casting methods. The hypereutectic aluminum-silicon casting alloys have been expected to be applied in various fields due to low coefficient of thermal expansion, high modulus and good wear resistance, but in practice they are not applied. The main reason is that they contain coarse primary silicon particles which give a product having poor machinabilities and poor mechanical properties. Thus, for improving the machinability and the mechanical strength, the refinement of the primary silicon particles in the hypereutectic aluminum-silicon casting alloy is effected by adding a modifier for refining the primary silicon particles, particularly such a modifier containing phosphorus. Unfortunately, the addition of the modifier for refining the primary silicon particles cannot give the well-refined primary silicon particles. Especially when the hypereutectic aluminum-silicon casting alloy contains 20% by weight or more of silicon, the coarse primary silicon particles are found.
Recently, it has been proposed to produce the hypereutectic aluminum-silicon alloy by a rapid solidification method. According to this method, the hypereutectic aluminum-silicon alloy with refined primary silicon particles can be obtained, even if it contains 20% by weight or more of silicon. In this case, the improvement of the machinability is satisfactory to a certain extent, but the improvement of the mechanical properties is limited. The addition of the modifier for refining the primary silicon particles could not give the satisfactory improvement of the mechanical properties.
An object of the present invention is to provide the hypereutectic aluminum-silicon alloy produced by the powder metallurgy technique, which contains the well-refined primary silicon particles.
Another object of the present invention is to provide the hypereutectic aluminum-silicon alloy produced by the powder metallurgy technique, which is excellent in machinability and mechanical properties.
The present inventors found that the reason of obtaining the insufficient improvement of the mechanical properties, especially the mechanical strength in the hypereutectic aluminum-silicon alloy produced by the powder metallurgy technique even if the modifier for refining the primary silicon particles in adequate amount is added is to present 0.03% by weight or more of calcium as an impurity therein, said calcium being derived from aluminum and silicon raw materials.